1. Field of the Invention
The present invention relates to an improved quantum nanostructure semiconductor laser that is particularly suited for application to distributed feedback (DFB) semiconductor lasers.
2. Description of the Prior Art
Distributed feedback semiconductor lasers provided with a periodic gain or refractive index structure along the waveguide, are important in future wavelength multiplex communication systems due to the fact that it is possible to closely control their lasing wavelength and that they can readily be integrated since, unlike in the case of a Fabry-Perot laser, they do not require a cleaving step.
For future progress, the manufacturing process has to be simplified. In the conventional fabrication techniques, as described for example in “MOCVD regrowth over GaAs/AlGaAs gratings for high power long-lived InGaAs/AlGaAs lasers,” P. K. York, J. C. Connolly, et al., Journal of Crystal Growth 124 (1992) pp 709-715 (Reference 1), fabrication comprised forming a lower cladding layer on the substrate, followed by a lower guide layer, an active layer and an upper guide layer in a first crystal growth process, then forming the guide layer with a grating corresponding to the integer multiples of a quarter wavelength in the waveguide, then using a separate, second crystal growth step to form an upper cladding layer on the guide layer having a periodic structure.
In order to obtain a lateral confinement effect, in addition to these basic steps, a stripe was formed along the optical waveguide by etching or the like, using a mask of silicon dioxide or the like, a third growth step was used to form a current blocking layer buried in the sidewall of the stripe, and then the silicon dioxide mask used for the selective growth was removed and a fourth growth step used to form ohmic contacts and to planarize the structure.
However, the use of multiple lithography and crystal growth steps steeply increased the manufacturing cost, preventing it becoming widespread on an industrial scale. Moreover, recombination current was increased due to the regrowth interface being in the vicinity of the active layer, causing an increase in the threshold current. Thus, fundamentally there were limits to the technique, making it inapplicable, without modification, on a continuing basis into the future.
On the other hand, quantum nanostructure semiconductor lasers using quantum wires or quantum dots in the active region are suitable for realizing high-performance optical devices since the density of the states of the electron systems is concentrated at specific energy levels. There were also found to be other merits in addition to the initially-projected quantum effect. In the case of a quantum dot, for example, because the dot is surrounded by wide bandgap material, carriers can be confined inside the dot regardless of device configuration, making it possible to pursue the addition of functions without increasing the manufacturing cost, by just modifying the crystal growth conditions. With respect also to high-speed modulation, stable lasing properties can be predicted since the spatial movement of carriers is suppressed.
Generally, however, in order to realize stable, high-speed modulation, optical gain ratio in the main lasing mode to that in the side mode (the side mode suppression ratio) has to be large. In the case of conventional index-coupled distributed feedback lasers in which there are two equal longitudinal modes, a phase-shifted DFB laser has been devised by providing in the resonator center a phase shifter, the wavelength in which corresponds to ¼ the wavelength in the lasing medium.
However, index-coupled distributed feedback lasers produce an optical intensity distribution along the direction of the resonator, and as described in “Stability in Single Longitudinal Mode Operation in GaInAsP/InP Phase-Adjusted DFB Lasers,” Haruhisa Soda, et al., IEEE Journal of Quantum Electronics, Vol. QE-23, No. 6, June 1987 (Reference 2), lasing mode instability caused by spatial hole burning becomes a problem.
On the other hand, in the case of gain-modulated DFB lasers, only one lasing mode is possible and there is no intensity distribution along the resonator direction, so that, as recognized in, for example, “Enhanced Performance of Uncooled Strongly-Gain-Coupled MQW DFB lasers in 10 Gb/s Link Applications,” S. Yang, et al., ECOC 2001 Proceedings, 27th European Conference on Optical Communication, Vol. 2, pp 124-125 (2001) (Reference 3), a good-quality dynamic single-mode can be obtained. Stabilizing the main lasing mode and simplifying the manufacturing process will be very promising with respect to the construction of semiconductor lasers and the like for the widespread implementation of fiber to the home.
It would be best if the high-density, uniform integration of quantum wires and quantum dots in a positional relationship, based on specific rules, could be achieved with one-time crystal growth. That would make it possible to rationally realize wavelength-stabilized semiconductor laser modules and semiconductor saturable absorbers required for self-starting of ultrafast solid-state lasers.
However, the upper and lower cladding layers have to be at least 0.5 to 1 μm thick to confine the light in the optical waveguide. Thus, if, after the grating has been formed on the substrate and a lower cladding layer of that thickness has been formed, it were possible to maintain a satisfactory grating configuration on the surface of the lower cladding layer, it would be possible to form an active layer adjacent to the cladding layer grating using one-time crystal growth, markedly simplifying the process of manufacturing distributed feedback semiconductor lasers.
Based on this thinking, in Japanese Patent No. 3536978 (Reference 4) the present inventors disclosed a technology for growing V-grooves 1 μm or more in the thickness direction while maintaining a good shape by setting the initial shape of the compound semiconductor substrate and a crystal growth temperature suitable for the composition ratio of the epitaxial layer. By using this technology, the inventors were able to use one-time MOCVD to form a periodic structure comprised of quantum wires disposed orthogonally to the semiconductor waveguide at periods that were integer multiples of ¼ the wavelength in the lasing medium, making it possible to realize a gain-coupled DFB laser in which mode competition could not readily occur.
Moreover, while not yet public knowledge at the time of the present patent application, in Patent Application No. 2002-51548 (Reference 5) the present inventors realized a structure in which both ends of quantum wires were confined using a one-time growth process, by forming quantum wires having a finite-length corresponding to the stripe width of the semiconductor laser. The problem of carrier leakage along the wires outside the optical resonator in the case of the usual quantum wire formation orthogonal to the optical waveguide could be resolved, in accordance with the technology disclosed by the application, to use cladding layers to enclose the ends of the quantum wires. The finite-length quantum wires satisfy the carrier confinement effects like quantum dots, while at the same time, polarization and position controllability which is inherent from the original infinite-length quantum wires.
There have been many reports relating to forming an optical waveguide structure and current blocking structure by the least possible number of crystal growth steps to achieve the goal of simplifying the fabrication process and lowering the threshold current. In Japanese Patent No. 2716693 (Reference 6), for example, in the selective MOCVD growth on a ridge substrate, the fact that the growth rate of higher-order planes formed on sidewalls is slower than the growth rate on the substrate (100) plane was utilized for one-time growth formation of a buried Fabry-Perot laser. Also, in Japanese Patent No. 2081665 (Reference 7), the present inventors disclosed a technology for forming a quasi-buried Fabry-Perot laser by using a dielectric window to limit the growth region.
It is possible to use a ridge substrate to form a buried heterostructure such as is disclosed in Reference 6, but it is difficult to suppress current feeding through the current blocking layers due to the thyristor effect. This can be improved such as by providing a p-type current blocking layer beforehand on an n-type substrate, as recognized in JP-B HEI 05-55692 (Reference 8), and by holding down the concentration of the n-type blocking layers, as in JP-A 2003-234543 (Reference 9), and selectively diffusing the cap layer stripe portion and forming a p-type high-concentration contact layer. In the future, high-speed modulation characteristics will be important, for which it will be important to reduce parasitic capacitance. For this, JP-A 2003-243774 (Reference 10) discloses a technology in which films of oxidized AlAs are used for current blocking.
However, the techniques disclosed in the references described in the above are undesirable in that they complicate the device manufacturing process. Using regrowth to form a current blocking structure on sidewalls requires extra work and trouble. Even in the case of the technique disclosed by Reference 5, as a further development of Reference 4, which would seem to be the most desirable at this point in time, it is difficult to control the etch depth used to form the ridge waveguide after the crystal growth, making it difficult to obtain reproducibility of the characteristics.
The object of the present invention is to provide a quantum nanostructure semiconductor laser having a low threshold current and stable lasing wavelength with simple and reproducible processing steps, using one-time crystal growth on a non-planer substrate to form an optical waveguide and a current blocking structure having a high current blocking effect.